Picking Apart Pluto: Learn Facts About Pluto, the Former Ninth Planet

It’s time to learn about Pluto, the former ninth planet. Why is Pluto not a planet anymore, per the scientific definition? Because it only meets two of the required criteria used for planetary classification. That change is an example of how the science facts you may have learned in school may occasionally change based on new information. Even so, there are still plenty of fascinating things to learn about this former planet.  What are some interesting facts about Pluto that you need to know? Let’s dive into them now. 

10 Interesting Facts About Pluto

1. It Rotates Backward

On Pluto, the sun rises in the west and sets in the east, the exact opposite of what it does here on Earth. That is due to the fact that it, along with Venus and Uranus, rotates in a retrograde fashion, unlike the rest of the planets in the solar system. 

2. Sometimes, It’s The Eighth Planet

Pluto doesn’t always stay on the outskirts of the solar system. Its orbit is so eccentric that it ranges from 4.4 billion to 7.3 billion kilometers away from the sun. Sometimes, that means it’s closer to the sun than Neptune, making it the eighth planet. 

3. It Was a Planet for 76 Years

Astronomers discovered Pluto in 1930 and for 76 years, it was the 9th planet in our solar system. Unfortunately, in 2006 it lost that title when the IAU changed what defined a planet. It’s still considered a dwarf planet, and anyone who was raised to memorize the nine planets in the solar system still thinks of it as a planet in its own right. 

4. We’ve Been to Pluto Once

We haven’t sent a lot of spacecraft to explore the outer reaches of the solar system, but we have been to Pluto once. NASA launched New Horizons in 2006, and it reached the dwarf planet in 2015, sending us back some of the best pictures of Pluto to date. It even found a massive heart-shaped plain on the planet’s surface, which just made us love Pluto that much more.

5. It’s the Largest Object in the Kuiper Belt

While Pluto might be considered a dwarf planet in our solar system, it is the largest object in the Kuiper Belt, part of the Trans-Neptunian region that makes up everything beyond the planet Neptune. There are a number of other dwarf planets in the Kuiper Belt, but Pluto is currently the largest. 

6. It Occasionally Has an Atmosphere

While it’s not something that you’d be able to breathe, the dwarf planet does occasionally have an atmosphere. When it moves closer to the sun, the star’s warmth heats up frozen carbon monoxide, methane and nitrogen ice on the planet’s surface. That sublimates to gas and becomes something of an atmosphere for the dwarf planet. When it moves further away, the atmosphere freezes and falls back to the rocky surface. 

7. It’s Named for the Roman God of the Underworld

Pluto gets its name from the Roman God of the Underworld, but it wasn’t named by the astronomer who discovered it. The name came as a suggestion from a schoolgirl named Venetia who was a fan of classical mythology. On hearing that astronomers had found a new planet, suggested the name Pluto and the rest, as they say, is history.

8. Its Moons Are Named After Characters Associated With the Underworld

Pluto has five moons — Charon, Nix, Hydra, Styx, and Kerberos — are named after different icons in Greek and Roman mythology. Styx is one of the rivers that surround the underworld. Charon is the ferryman who brings souls across the river Styx. Nix is the Roman goddess of darkness and night, and Kerberos is the roman version of Cerberus, the three-headed dog that guards the gates of the underworld. Finally, the Hydra is one of the monsters in Greek and Roman mythology, a multi-headed serpent that Hercules fought. 

9. It Is Considered a Double-Planet System

Most of Pluto’s moons are small, but Charon is so large that some experts consider Pluto and Charon the solar system’s only double-planet system. They’re also mutually tidal-locked, meaning the same surfaces of each planetary body face each other at all times. Charon orbits Pluto once every 6.4 days. 

10. Astronomers Predicted Pluto 15 Years Before They Found It

An astronomer named Percival Lowell started to see hints that there might be another planet beyond Neptune in 1905. He predicted Pluto’s existence in 1915 but unfortunately died before he was able to make the official discovery. We didn’t find Pluto until 1930, 15 years after Lowell’s original prediction. 

Relatively New Facts About Pluto to Ponder

Many people who think back to what they last heard about Pluto immediately recall the buzz surrounding its reclassification. Why is Pluto not a planet, despite what they learned in science class? Some may have even found old science materials that treated Pluto as a planet, viewing them as relics from a different time. However, no matter what you may think, Pluto remains in the news regularly because scientists still study it and know there is always more to discover. Here are some exciting examples. 

1. Scientists Solved Some Mysteries of Pluto’s Heart-Shaped Structure

Pluto’s surface has a heart-shaped structure, and its origins have puzzled researchers until recently. However, an international team of astrophysicists answered some of the associated burning questions by creating several numerical simulations of the teardrop-shaped part of the heart. 

That approach allowed them to conclude that a cataclysmic event in Pluto’s early history caused that section of the heart shape. Additionally, the group’s findings suggest Pluto’s surface is different than previously thought. More specifically, it does not have a subsurface ocean. Another interesting fact about Pluto’s “heart” is that it has a white nitrogen ice filling that convects and moves to maintain a continually smooth surface. 

2. New Additions Made to Pluto’s Chemical Inventory

As people learn more facts about Pluto, they can often expand previously gathered knowledge. Scientists did that when observations from the James Webb Space Telescope showed hydrogen peroxide and carbon dioxide on the frozen surface of Pluto’s largest moon, Charon, giving them a broader picture of the chemical inventory on Charon.

Although previous investigations into Charon confirmed the presence of water, ice, organic materials and ammonia-bearing species, this was the first time scientists used the observations to verify that carbon dioxide and hydrogen peroxide are there, too. 

Additionally, the group said the telescope’s observations allowed them to study the scattering of wavelengths from Charon’s surface at longer wavelengths than they previously could. This is an example of how ongoing scientific progress can reveal new, relevant information, keeping scientists and the public motivated and interested. 

3. Pluto Is Arizona’s State Planet

Why is Pluto not a planet? People curious about the answer likely never considered how one state would still define it as one. In March 2024, Arizona’s governor signed a law that made Pluto the state’s official planet. Arizona also has a state dinosaur and state flower, among other items, so this classification is not entirely surprising, besides the fact that Pluto no longer has its former planetary status. 

Indeed, Arizona has a long history of furthering planetary exploration, particularly since many astronauts stop in Flagstaff as part of their training. Also, people commenting on the matter clarified that Pluto is still a planet but a dwarf one rather than a giant. 

Pluto Properties & Information

• Location in the Solar System: Formerly the 9th planet, now considered a dwarf planet.
• Distance From the Sun: 5,874,000,000 km or 39.28 AU
• Composition: 70% rock, 30% water ice
• Size: 1,151 km or 715 miles at the equator
• Surface: Rocky surface, covered with ice made from methane, nitrogen and CO2.
• Structure: Rocky core with a mantle of water ice, and a surface of nitrogen ice.
• Color: Black, orange and white
• Atmosphere: Occasionally has a thin nitrogen atmosphere
• Moons: 5
• Temperature: -380.2°F (-229°C)
• Orbital Period: 248 Years
• Rotation Period: 153 hours

Who Discovered Pluto?

Astronomers didn’t discover Pluto until 1930. Clyde Tombaugh, an astronomer at the Lowell Observatory — named for Percival Lowell, who first predicted the location of the eccentric dwarf planet — finally located Pluto. Tombaugh and his team used Lowell’s calculations in order to finally locate the last planet in our solar system. Lowell, while observing the orbits of Neptune and Pluto, noticed that something else was tugging at the ice giant’s orbits. That simple observation was enough to help the Lowell Observatory team figure out where Pluto was hiding. 

What Is Pluto Known For?

Pluto is probably best known for being the only planet in the solar system to actually lose its planetary status. In 2006, the International Astronomical Union changed what it takes to be defined as a planet. As of 2006, a celestial body needs to meet three criteria to be considered a planet: 

1. It has to orbit a star. 
2. It has to be big enough that its gravity forces it into a spherical shape.
3. It has to be big enough that its gravity gets rid of anything of similar size in its orbit. 

When the IAU updated its definition, Pluto didn’t meet the criteria, losing its planetary status and becoming a dwarf planet — a new definition for small celestial objects too big to be a solar system body but too small to be a planet. Other dwarf planets include Ceres in the Asteroid Belt, and Kuiper Belt Objects Eris, Haumea and Makemake. Experts estimate that there may be upwards of 100 more dwarf planets hiding in the Kuiper Belt waiting to be discovered. 

What Makes Pluto Unique?

Pluto might be a dwarf planet but there are plenty of things that make it unique. For example, it takes the planet 248 years to create one orbit. It’s also on a different plane than the rest of the planets. Instead of orbiting alongside the rest of its celestial neighbors, its orbit is on a 17-degree incline. 

Its orbit also carries it closer to the sun than Neptune sometimes, making it the eighth planet instead of the ninth. In spite of being an otherwise barren rock, Pluto also occasionally develops an atmosphere. As it moves closer to the sun, the ice on its surface begins to melt, sublimating to gaseous carbon monoxide, nitrogen and methane, creating a kind of atmosphere. As it moves away from our home star, those gasses freeze again, falling back to the surface. 

Beyond Facts About Pluto

This brings us to the end of our exploration of Pluto. Our solar system stretches beyond the planets, through the Kuiper Belt and the Oort Cloud, all the way to the edge of the heliosphere before we start heading out into interstellar space. There is so much of the universe that we haven’t even been able to start exploring, and there are a lot of amazing things out there that are just waiting to be discovered.

Editor’s note: This article was originally published on October 8, 2020 and was updated November 28, 2024 to provide readers with more updated information.